1. Field of the Invention
The present invention relates to a magnetron for used in equipment using microwaves such as a microwave oven.
2. Description of the Related Art
FIG. 15 is a longitudinal section view of a general magnetron which is conventionally used in a microwave oven, and FIG. 16 is an enlarged section view of the main portions of the magnetron shown in FIG. 15. In FIGS. 15 and 16, in the inside of a cylindrical-shaped anode barrel member 10, there are radially disposed anode vanes 11, while spaces respectively enclosed by the mutually adjoining anode vanes 11 and anode barrel member 10 constitute a cavity resonator. In the central portion of the anode barrel member 10, there is disposed a cathode structure member 12, while a space enclosed by the anode structure member 12 and anode vane 11 constitutes an action space 13. On the upper end of the anode barrel member 10, there is fixedly mounted a pole piece (which is hereinafter referred to as an output side pole piece) 14, whereas, on the lower end thereof, there is fixedly mounted another pole piece (which is hereinafter referred to as an input side pole piece) 15.
The output side pole piece 14 is formed in a funnel shape by drawing a magnetic plate member having small magnetic resistance such as an iron plate member. That is, the output side pole piece 14 provides a funnel shape which includes a small-diameter flat portion FL1 having a penetration hole 14A formed in the central portion thereof, a large-diameter flat portion FL2 having a larger diameter than the small-diameter flat portion FL1, and a conical-shaped slanting portion SL which connects together the large-diameter and small-diameter flat portions FL2 and FL1. In the output side pole piece 14, besides the penetration hole 14A formed in the central portion thereof, there is also formed another penetration hole 14B through which an antenna 16 can be penetrated.
The input side pole piece 15, similarly to the output side pole piece 14, is formed in a funnel shape by drawing a magnetic plate member having small magnetic resistance such as an iron plate member. That is, the input side pole piece 15 provides a funnel shape which includes a small-diameter flat portion FL1 having a penetration hole 14A formed in the central portion thereof, a large-diameter flat portion FL2 having a larger diameter than the small-diameter flat portion FL1, and a conical-shaped slanting portion SL which connects together the large-diameter and small-diameter flat portions FL2 and FL1. Just above the output side pole piece 14, there is disposed a metal ring 17 which covers the output side pole piece 14, while, just below the input side pole piece 15, there is disposed a metal ring 18 for covering the input side pole piece 15. Just above the metal ring 17 and just below the metal ring 18, there are respectively mounted ring-shaped magnets (not shown) in a close contact manner, the central portions of both of which are formed hollow. To the cathode structure member 12, there is connected a lead 19 which is used to apply a direct current voltage to the cathode structure member 12.
When using the conventional magnetron, after the inside of the magnetron is evacuated, a direct current high voltage is applied to between the anode vane 11 and cathode structure member 12. In the action space 13, there is formed a magnetic field due to the two magnets (not shown). When the direct current high voltage is applied to and between the anode vane 11 and cathode structure member 12, electrons are drawn out from the cathode structure member 12 and thus they fly out toward the anode vane 11. At the then time, the magnetic field due to the two magnets (not shown) concentrates in a gap existing between the output side pole piece 14 and input side pole piece 15, and it acts on the action space 13 in a direction perpendicular to a direction where the cathode structure member 12 and anode barrel member 10 are opposed to each other. As a result of this, electrons flown out from the cathode structure member 12 are rotated and moved in a spiral by a force which is generated by the magnetic field due to the magnets (not shown), and the electrons finally arrive at the anode vane 11. Energy generated due to the then time electrons movements is applied to the cavity resonator to contribute toward the oscillation of the magnetron.
By the way, when discharging the air existing in the inside of the magnetron, the air on the input side, as shown in FIG. 17, passes not only through a penetration hole 15A opened up in the central portion of the input side pole piece 15 but also through a penetration hole 21A opened up in a lower end hat 21 which constitutes the cathode structure member 13. Since the lower end hat 21 is situated in the penetration hole 15A of the input side pole piece 15 and one end portion of a filament coil 22 is situated in the penetration hole 21A of the lower end hat 21, the portions of the penetration holes 15A and 21A, through which the air passes, are made narrow. This makes it impossible to provide a large air discharge conductance (an air exhaust efficiency), thereby taking much time to discharge the air. Owing to the fact that it takes much time for the air exhaust, there is a fear that there can occur a poor degree of vacuum. To solve this problem, there is proposed a structure in which an output side pole piece having a penetration hole 14B, through which the antenna 16 is to be passed, is employed as an input side pole piece to thereby increase the air discharge conductance (for example, see Japanese Utility Model Publication Sho-63-18745). The air, which has passed through the input side pole piece 15 and flowed into the inside of the anode barrel member 10, is discharged from an exhaust pipe 20 through the penetration hole 14A opened up in the central portion of the output side pole piece 14 as well as through the penetration hole 14B opened up for the passage of the antenna therethrough.
However, even when there is disposed a new opening in the input side pole piece 15 (there may also be the output side pole piece 14) in order to discharge the air on the input side with high efficiency, depending on the size of the opening, there is also a fear that the maximum magnetic field strength can be lowered or higher harmonic waves can leak.